Pagination not final (cite DOI) / Pagination provisoire (citer le DOI) 1 ARTICLE The age of the Tseax volcanic eruption, British Columbia, Canada Glyn Williams-Jones, René W. Barendregt, James K. Russell, Yannick Le Moigne, Randolph J. Enkin, and Rose Gallo Abstract: A recent volcanic eruption occurred at Tseax volcano that formed a series of tephra cones in northwestern British Columbia, Canada. The explosive to effusive eruption also formed a 32 km long sequence of Fe-rich Mg-poor basanite–trachy- basalt lavas covering ϳ40 km2. Oral histories of the Nisg_a’a Nation report that the eruption may have caused as many as 2000 fatalities. The actual eruption date and question of whether there was one or multiple eruptive episodes in the 14th and 18th centuries are, as of yet, unresolved. New radiocarbon dating of wood charcoal from immediately beneath vent-proximal tephra deposits and complementary age information suggest an eruption in 1675–1778 CE (95.4% probability) was responsible for the formation of the tephra cone. New paleomagnetic and geochemical data from the tephra cone and lava flows suggest there is, in fact, no statistically significant difference in time between the explosive and effusive deposits and that they formed during a single eruptive episode. Key words: Tseax volcano, lava flow, tephra cone, paleomagnetism, radiocarbon dating, geochemistry. Résumé : Une éruption volcanique récente a eu lieu au volcan Tseax, qui a produit une série de cônes de téphra dans le nord-ouest de la Colombie-Britannique (Canada). L’éruption explosive à effusive a également produit une séquence longue de 32 km de laves de basanite–trachybasalte riches en Fe et pauvres en Mg couvrant ϳ40 km2. Des récits oraux de la Nation Nisg_a’a mentionnent que l’éruption pourrait avoir causé jusqu’à 2000 morts. Les questions du moment exact de l’éruption et à savoir si elle comportait un seul ou plusieurs épisodes éruptifs aux 14e et 18e siècles demeurent à ce jour sans réponses. De nouveaux résultats de datation au carbone radioactif de charbon de bois prélevé immédiatement sous des dépôts de téphra proximaux à la cheminée et de l’information temporelle complémentaire indiqueraient qu’une éruption vers 1675–1778 EC (probabilité de 95,4 %) est à l’origine de la formation du cône de téphra. De nouvelles données paléomagnétiques et géochimiques sur le cône de téphra et les coulées de lave donnent à penser qu’il n’y a en fait aucune différence statistiquement significative dans le temps entre les dépôts explosifs et effusifs et qu’ils se sont formés durant un seul épisode éruptif. [Traduit par la Rédaction] Mots-clés : volcan Tseax, coulée de lave, cône de téphra, paléomagnétisme, datation au carbone radioactif, géochimie. For personal use only. Introduction Le Moigne et al. 2020), petrological and geochemical studies Tseax volcano situated in northwestern British Columbia, Canada (Nicholls et al. 1982, 1997; Higgins 2009; Gallo 2018), and dating (55.11085°N, 128.89944°W), comprises several small tephra cones (Lowdon et al. 1971; Symons 1975; Wuorinen 1978; Higgins 2009). and a 32 km long basanite–trachybasalt lava (e.g., Hanson 1923; The Tseax vent area comprises a number of short eruptive fis- Sutherland Brown 1969). The eruption is believed to have occurred in sures and two small tephra cones; the larger of the two cones is the 1700s (e.g., Lowdon et al. 1971; Higgins 2009) and to have de- partially enclosed by a spatter rampart described previously as a dissected tephra cone (Sutherland Brown 1969; Wuorinen 1978). stroyed at least three Nisg_a’a Nation villages (Laxksiluux, Laxksi- wihlg_est, and Ts’oohlts’ap; Fig. 1) located on the banks of the Nass River, This original description, along with radiocarbon dating of a tree ϳ20 km from the volcano. The eruption may have caused as many as from the spatter rampart, raised the possibility of two distinct 2000 fatalities (Nisg_a’a Nation 2004). These fatalities would make it eruptive episodes occurring in 1325 CE and 1700 CE (Wuorinen Canada’s second-worst recorded natural disaster (Hickson and 1978). Recent field work has established the relative sequence of Edwards 2001) after the near contemporaneous Newfoundland volcanic events (Le Moigne et al. 2018, 2020), but the published hurricane of 1775 that caused at least 4100 deaths (Ruffman 1996). radiocarbon dating is sparse (three samples), leaving ambiguity There are no direct written accounts of the Tseax event; however, concerning the absolute timing and duration of eruptive activity. the rich oral history (adaawak - traditional histories) from the Importantly, the question of whether the volcanic field represents a monogenetic or a polygenetic system (e.g., Németh and Nisg_a’a people provides important observational data for the Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Dr. Glyn Williams-Jones on 06/29/20 eruption (Nisg_a’a Nation 2004; see Appendix A). Previous studies Kereszturi 2015), which may have had more than one eruption, have included volcanological and geomorphological mapping has important implications for future activity and hazard mitiga- (Hanson 1923; Sutherland Brown 1969; Roberts and McCuaig 2001; tion efforts and thus merits further study. Received 18 December 2019. Accepted 25 March 2020. G. Williams-Jones. Centre for Natural Hazards Research, Department of Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada. R.W. Barendregt. Department of Geography, University of Lethbridge, Lethbridge, AB T1K 3M4, Canada. J.K. Russell and R. Gallo. Department of Earth, Ocean and Atmospheric Sciences, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Y. Le Moigne. Centre for Natural Hazards Research, Department of Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada; Laboratoire Magmas et Volcans, Université Clermont Auvergne, 63178 Aubière, France. R.J. Enkin. Paleomagnetism and Petrophysics Laboratory, Geological Survey of Canada - Pacific, Sidney, BC V8L 4B2, Canada. Corresponding author: Glyn Williams-Jones (email: [email protected]). Copyright remains with the author(s) or their institution(s) and © Her Majesty the Queen in right of Canada 2020. Permission for reuse (free in most cases) can be obtained from copyright.com. Can. J. Earth Sci. 00: 1–16 (0000) dx.doi.org/10.1139/cjes-2019-0240 Published at www.nrcresearchpress.com/cjes on 30 March 2020. Pagination not final (cite DOI) / Pagination provisoire (citer le DOI) 2 Can. J. Earth Sci. Vol. 00, 0000 Fig. 1. Map showing Tseax cone (red circle) and four valley-filling lava flows; two early stage phenocryst-poor pahoehoe (light and dark green) and two later stage phenocryst-rich `a`a lavas (yellow and orange). RBn (white diamonds) denote locations of five new palaeomagnetism sites and black circles are sites from Symons (1975). Orange hexagon indicates dendrochronology study site (Hanson 1923) and green triangles show locations of radiocarbon sample sites. Light orange squares and circles show locations of pre-eruption Nisg_a’a villages (V1, Laxksiluux; V2, Laxksiwihlgest; V3, Ts’oohlts’ap) and fish camps (F1) / smokehouses (F2), respectively. Gennu Axwt (purple triangle) is a ridge from which scouts were said to have observed the eruption of Tseax (Nisg_a’a Nation 2004). Modified from Le Moigne et al. (2020). See Table 1 and Appendix Table A2 for sample locations. Inset map shows Tseax (red star) in the context of Neogene–Quaternary volcanic centres and complexes in the Canadian Cordillera, including the Garibaldi Volcanic Belt (GVB), the Wells Gray – Clearwater volcanic field (WGC), the Anaheim Volcanic Belt (AVB), the Wrangell Volcanic Belt (WVB), and the Northern Cordilleran Volcanic Province (NCVP). Modified after Edwards and Russell (2000). Map produced using ESRI ArcGIS 10.7. [Colour online.] For personal use only. Here we present new geochemical, paleomagnetic, and radio- Tseax volcano carbon data for samples collected from the Tseax volcanic depos- Tseax volcano, Wil Ksi Baxhl Mihl, formerly known as Aiyansh its (Figs. 1, 2) supported by detailed mapping of the tephra cones volcano, Aiyansh River volcano, or Tseax River cone, is located in Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Dr. Glyn Williams-Jones on 06/29/20 and lava flows (Gallo 2018; Le Moigne et al. 2018, 2020). The new the Anhluu’t’ukwsim Laxmihl Angwiga’asanskwhl Nisg_a’a (Nisg_a’a Me- radiocarbon dates derive from samples of charred wood collected morial Lava Bed Provincial Park) near Gitlaxt’aamiks (formerly New from beneath explosive tephra deposits ϳ890 m northwest of the Aiyansh) and Gitwinkshilkw (formerly Canyon City), ϳ60 km north- main tephra cone (Fig. 2). The paleomagnetic data sets derive from west of Terrace in northwestern British Columbia, Canada (Fig. 1). the lava field, tephra cone, and spatter rampart. Our sampling Tseax (pronounced see-ax) is the southernmost volcanic centre of campaign was specifically designed to test whether all the Tseax the Northern Cordillera Volcanic Province (Edwards and Russell volcanic deposits, representing explosive and effusive activity, 2000) and is notable for a 32 km long basanite–trachybasalt lava share a common paleomagnetic direction acquired during cool- flow (approximately 0.5 km3 covering ϳ36 km2; Fig. 1). More im- ing. The implication would be that all events have the same pa- portantly, it is the site of the second-youngest (after Lava Fork in leomagnetic age. The alternative is that the apparently earlier ϳ1800 CE; Elliott et al. 1981) and the deadliest volcanic eruption in stratigraphic unit (i.e., the partially dissected spatter rampart; Canada (Hickson and Edwards 2001). The tephra cones and valley- Fig. 2) has a different paleomagnetic direction and age than the filling lavas overlay intercalated sandstone to siltstone and mud- stratigraphically youngest units (i.e., tephra cone and the valley- stone from the Late Jurassic Bowser Lake group (van der Heyden filling lavas), as suggested from 14C dating by Wuorinen (1978). et al. 2000; Evenchick et al.